The present invention relates to a stator having a molded casing and, more particularly, to a stator for small motors having a first molded layer on a stack and a second molded layer enclosing a coil and portions of the first molded layer. The stator has applications in small motors such as spindle motors or servo motors, which are used in hard disk drives, wherein thin insulation is required.
The ever increasing demand for hard disk drives with greater storage capacity has resulted in increasingly strict requirements for cleanliness of the driving unit. Motors have been found to be a source of particles, gas, organic substances, and ionic substances. Thus, spindle motors used in hard disk drives must not produce contaminants during operation. It has been found that particles are generated from the stator or the wound windings of the rotor coil due to abrasion. Improvement in the functioning of hard disk drives is achieved by enclosures for containing particles and gases generated by the coil and stack of the motor stator of the hard disk driving unit. Even trace foreign particles can adhere onto the magnetic head or hard disk and cause noise or a head crash. Therefore, it is necessary to prevent particles or gasses from being generated by the-motor.
Various kinds of small or thin motors are applied in various fields, and the demand for further reduction in size is increasing. At the same time, simplification of the manufacturing process for such motors and improvements in reliability are required. Motor characteristics such as starting torque and power are improved when an insulator on the stack of the stator is thinner because more coil can be wound on the stator. Furthermore, a thinner insulation results in better thermal transfer characteristics allowing improved dissipation of heat generated by the motor. Finally, it is also necessary to protect against vibration, noise caused by electric current pulses and to increase durability of the hard disk unit.
In the conventional methods used to manufacture stators, multiple plates of punched thin thickness steel are interlocked to produce a lamination stack. The stack is then coated with insulation after an anti-corrosion treatment. Following the coating, a coil is wound after a winding guide is affixed. However, due to laminating the punched thin steel, the outer surface and slot being exposed is not be completely planar, thus resulting in recesses and projections.
A major issue for using an insertmolding process is the variation in the thickness of the laminated stack. Therefore, in order to perform insertmolding, screening of the thickness and adjustment of the core in the molding tool is required. The adjustment of the core in the molding tool further causes the formation of flash which requires deburring. Thus, the screening, the adjustment and the deburring add further costs to the production process.
A coating of 50-80 micron thickness is necessary to provide insulation on such rough surface. Prior to coating, costs are further increased by the requirement for a pre-treatment of shotblasting or tumbling to effect deburring. Despite such steps, the coating is often left with pinholes which deteriorate the insulation or edges are left uncovered by the coating which results in low yields.
The manufacturing process is also complicated because the winding guide is affixed after the coating is applied. There is a method disclosed by published Japanese patents #63-3636 and #63-3637 which uses a steel lamination with thermoplastic synthetic resin pre-adhered on outer face surfaces of the two outer most steal plate laminations. After lamination of the steel plates, the synthetic resin is heated to deform and flow to coat edges of the laminations to provide insulation. However, a sufficient amount of synthetic resin on the steel plate laminations must be provided to permit the requisite amount of deformation and coating using such a method. Accordingly, the resin coating must be relatively thick to permit a sufficient flow to the center of the laminations for complete coverage with synthetic resin. Furthermore, it is rather difficult to coat the whole stack with uniform thickness.
Methods to enclose wound wire with synthetic resin have been suggested. When enclosing windings with synthetic resin, it is important not to damage the insulation of the wire by heat or pressure, and not to disturb the wound coil. In general, enclosing with synthetic resin is performed with thermosetting resin under low temperature (less than 120.degree. C.) and low pressure. Bulk molding compound (BMC) is mainly used as resin material. The resin temperature is low, but the manufacturing cycle takes several minutes using such a method.
Some methods using injection molding have also been suggested for enclosing wound wire with synthetic resin. Published Japanese patent #61-10949 discloses inserting coiled wire into a molding tool for injection molding, setting the tooling temperature at a 120-150.degree. C., using polyphenylenesulfide (PPS) resin with 20-40 wt % inorganic filler, and setting the resin temperature at 300-350.degree. C. and injection pressure at 800-1000 kg/cm.sup.2 in the process of coating the wound wire. This enables molding with relatively short cycle by using thermoplastic resin for enclosing wound wire since conventionally used thermosetting epoxy resin requires a longer molding cycle which results in low productivity. Since this method applies high pressure to inject molten resin at high temperature, it may be acceptable for applications where the diameter of wire is large enough and insulating coating of coil is thick enough to withstand the process. In applications involving coil wire of smaller diameter and a thin insulating coating of the coil wire, the wound coil is susceptible to being disturbed and the insulation coating of wire is prone to damaged. Thus, this method is not practical in such applications.
Published Japanese patent #3-70441 discloses a stack having multiple slots to be wound. The stack of the rotor with a rectifier and an outer surface of wound wire are enclosed by injection molding with insulating encapsulating material (polyacetal with glass fiber). The resin injecting position is located at an opposite surface of the rectifier and parallel with the axis of the rotor. Injection is performed in two stages. In the first stage an injection pressure of is 220 kg/cm.sup.2 is used and in the second stage a pressure of 50 kg/cm.sup.2 is used. However, the method relates to the rotor, not the stator, and differs from the object of the present invention. The technical point of the published patent is to specify the direction of resin injection to prevent disturbance of wound wire and minimize molding defects such as wrinkles and sink marks. The resin injection time is reduced to 2.5 seconds. Even with this shortened time, as the insulation coating of the winding of the stator for the small-sized precision motor is so thin, the wire is prone to damaged by the high temperature of molten resin. Also, as the initial injection pressure of 220 kg/cm2 is high, the wound coil is prone to being disturbed by the effect of injected resin.
Finally, published Japanese patent #6-327208 discloses an assembly of a stator block of an axial gap type DC brushless motor provided with multiple stator blocks having stacks of columnar soft magnetic material with coils wound between a set of permanent magnets affixed to the rotor. The coils are fixed by a resin having a major filler with a heat conductivity of more than 10 (w/m.multidot.k). The resin to be used is either thermoplastic and thermosetting resin, but the thermosetting resin is mainly described. Molding methods discussed include injection molding, which is not disclosed in detail, a potting method for liquid thermosetting resin, a transfer molding method for powder thermosetting resin, and a casting method, the particular method depending on the configuration. An epoxy resin containing a filler of improved heat conductivity, such as aluminum oxide or aluminum nitride, is also discussed but no reference to the flow character of such a resin is made.